Field of the Invention
The present invention relates to the field of Lithium ion batteries.
Related Art
As an important candidate for electric vehicle (EV) and hybrid electric vehicle (HEV) power sources, lithium-ion batteries based on graphite anodes and ethylene carbonate (EC) containing electrolytes have gained wide application. Conventional organic solvents comprise ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). Ethylene carbonate (EC) forms a stable solid electrolyte interphase (SEI) at ˜0.8 V before lithium intercalation. Being Li+ permeable and electronic non-conductive, the SEI prevents further electrolyte decomposition and allows reversible lithiation and delithiation of graphite anodes. The major disadvantage of EC is its high melting point at around 34° C., since EC is a solid material at room temperature EC needs other co-solvents such as dimethyl carbonate (DMC) and diethyl carbonate (DEC). The relatively high melting point of EC also limits the use of lithium-ion batteries at low temperatures. Propylene carbonate (PC) has a wide liquid temperature range (−48.8˜242.0° C.) and very good low temperature performance compared to EC. However, with only a negligible structural difference from EC, PC undergoes a detrimental solvent decomposition on the surface of graphite with high crystallinity. This causes disintegration of the graphite electrode, usually accompanied with delamination of the active material from a current collector and ultimately cell failure.
Two different physical pictures are commonly used to explain the detrimental effect of PC in a graphite lithium-ion battery. In the first scenario originally proposed by Peled and developed by Aurbach, a decomposition voltage of the cyclic carbonates is at approximately 0.8˜1.0 V higher than a lithium intercalation voltage 0.01˜0.25 V. In the case of an EC cyclic carbonate, the decomposition products form a surface film (SEI), which is compact and protective to prevent further solvent co-intercalation into graphene layers. However, in the case of an PC cyclic carbonate, a surface film formed by PC is not so effective, wherein repeated solvent co-intercalation occurs and the resulting decomposition products cause deterioration of the graphite capacity and reversibility. Besenhard and Winter proposed the formation of solvated graphite-intercalation compounds (GICs)-Li(solv)yCn. Co-intercalation of GICs and the subsequent decomposition products determine the cell behavior.
Based on Besenhard and Winter's solvent co-intercalation theory, the model in FIG. 1 is used to explain exfoliation of graphite in PC. Black dot represents Li+, the circles represent PC and hexylene carbonate (HeC) solvents. This model assumes a solvation number of 2 for the ease of illustration, although a more realistic value is about 3 or 4. When using pure PC as solvent as presented in FIG. 1, solvated Li+ tend to drag PC molecules into graphene layers in the process of intercalation. There are a lot of electrons in the graphite in the discharge (lithiation) process, which cause a two-electron reduction of PC. As shown in FIG. 1, each PC molecule consumes two electrons and decomposes to lithium carbonate and propene. The propene gas induces micro-cracks inside the graphite layers/particles which leads to a disintegration of the graphite electrode. GICs are stable enough to be detected by X-ray measurement. Yamada showed that PC exfoliation is prevented by changing PC/DMC ratio from 1/1 to 1/7, wherein a smaller ratio of PC molecules in the Li+solvation sheath contributes to this behavior. Cresce et al. recently used mass spectroscopy with a soft ionization technique electrospray ionization to study the Li+ solvation structure, which revealed a close connection between the SEI component and Li+ solvation structure. Chung et al. modified different parameters that could influence the solvent decomposition behavior in graphite half cell, the overall results show that solvent co-intercalation is critical to explain the cell behaviors.